This invention relates generally to rotor blades for use in turbine engines, and more specifically, but not by way of limitation, rotor wheel assemblies that promote efficient installation while also reducing certain types of wear.
As will be appreciated, turbine engines (for example, combustion or steam turbine engines) include flowpaths defined through turbine sections or turbines through which a pressurized working fluid is expanded during operation. Within such turbine, alternating rows of static nozzles or stator blades and buckets or rotor blades are axially stacked to interact with the flow of working fluid. The stator blades direct the flow of working fluid onto the rotor blades so to induce rotation about a central axis of the turbine. The rotor blades are connected to a rotor wheel that is connected to a shaft so that this rotation drives the rotation of the shaft, which then may be used to do work, for example, turn the coils of a generator.
Such turbines may include several stages or rows of rotor blades, and the size of these rotor blades generally increases as the rows progress in the downstream direction. The rotor blades within the later stages of the turbine engines, thus, typically have considerable length and weight. Along with the highly contoured shapes of these rotor blades, the considerable size creates certain geometrical or spatial restraints during installation, as well as particular structural and retainment issues for the rotor blades during operation.
Turbine rotor blades connect to the rotor wheel via particular types of connectors or connection assemblies. These typically include a particularly shaped root of the rotor blade—for example, a dovetail or “multi-tang” fir tree shape—that engages a correspondingly shaped slot formed through the outer perimeter of the rotor wheel. Such shaped connectors are effective at providing a number of stress-spreading contact surfaces between the root and the slot, and, once these contact surfaces engage, relative movement between the rotor blade and the rotor wheel is substantially restrained. According to certain conventional designs, such rotor blade and rotor wheel connections are often constructed with a certain degree of “wiggle room”, “play” or “excess room” in the radial direction, which allows some freedom of movement relative to the rotor wheel for rotor blades already engaged within the slot.
During high speed operation, it will be appreciated that such excess room in the radial direction does not lead to the rotor blade moving within the connector because the centrifugal forces drive the rotor blade in an outward radial direction and thereby fix it against the contact surfaces within the slot. As will be appreciated, the rotor blade will remain in this position as long as the turbine continues operating at high speed. However, during low speed operation, such as turning gear operation, the excess room allowed in the radial direction results in the rotor blade moving or jostling in this direction as it rotates. This movement is generally undesirable due to the wear it causes to the contact faces of the root and within the slot. However, having the excess room in the connector is nonetheless often necessary to facilitate assembly of the rotor blades. Specifically, some movement or “fanning” of rotor blades is needed during the assembly of the row. One of the reasons for this is that the outer tips of the airfoil of the rotor blade typically have interlocking features. Further, the airfoil portions of the rotor blades may overlap such that the assembly of the last rotor blades in the row is made difficult, if not impossible, unless a certain amount of movement is not maintained within the connectors.
Conventional technology includes the use of springs or, alternatively, overly tight dovetail fits, but each has limitations that are undesirable for use with the longer and heavier rotor blades in the later stages of the turbine. For example, springs may be used, but such springs and forces they needed to provide to radially secure the rotor blades are sizeable, particularly to overcome the 3-o'clock and 9-o'clock moment loading of the rotor blades during slow speed operation. Springs large enough to do this may limit the robustness of the rotor wheel design because the oversized dovetail bottoms required to accomplish this increases the stresses applied to the rotor wheel. Springs also are difficult to install and add to the complexity of the assembly. On the other hand, with overly tight connectors, the relative movement between the rotor blade and rotor wheel that is needed for efficient installation is eliminated.
Given these considerations, novel connection assemblies between rotor blades and rotor wheels, which permit some relative movement during installation but that may be made to restrain such movement once installation is completed, would have considerable economic value.